<?php
    
    // set date for calculation
    $day = 18;
    $month = 9;
    $year = 1972;

    $bdayunix = mktime(0,0,0,$month, $day, $year);
    $nowunix = time();
    $ageunix = $nowunix - $bdayunix;
    $age = floor($ageunix / (365*24*60*60));

    echo "Age is $age";
?>
